1. Field of the Invention
The present invention relates to multifilamentary superconductive wires clad in copper or a copper alloy and composed of filaments constructed of the superconductive, intermetallic compounds Nb.sub.3 Sn or V.sub.3 Ga, each having an A-15 structure, and additives from the group including rare earth metals, Th, U, Ti, Zr, Hf, V, Nb, Ta, Mo, Fe, Co, Ni, Pd, Cu, Ag, Al, Pt. The invention also relates to a process for manufacturing such superconductive wires.
2. Technology Review
Magnetic fields up to a field intensity of 15 Tesla can today be realized in commercial superconductive laboratory coils at temperatures of 4.2.degree. K. More recent developments have brought magnetic fields of 18 Tesla (at 1.8.degree. K. even up to 20 Tesla) within the realm of the possible. To produce such fields, materials such as Nb.sub.3 Sn, or V.sub.3 Ga (both with an A-15 crystal structure) must be employed. These, however, are all very brittle materials, having an elongation at rupture of about 0.1%. To be able to live with this unfavorable characteristic, a plurality of rather complex procedures has been developed for the production of the respective superconductive wires. In all of these processes, partial elements which do not yet exhibit the final superconductive characteristics but are still ductile, are mechanically deformed into a wire of the final dimensions. This wire is then subjected to a reaction heat treatment between about 550.degree. and about 800.degree. C. to form the superconductive phase. A typical superconductive wire of about 1 mm in diameter is composed of several thousand filaments composed of the above-mentioned A-15 materials. These filaments have diameters (or characteristic dimensions if they deviate from the circular shape) below 5 .mu.m, with individual manufacturing methods even producing typical filament dimensions of &lt;1 .mu.m.
Multifilamentary superconductive wires based on bronze-Nb.sub.3 Sn with ternary or quaternary additives of, for example, uranium, titanium, zirconium, hafnium, vanadium, tantalum, iron, nickel, palladium, aluminum or others, are disclosed in European published Patent Application No. 48,313. There, the additives are used to avoid the so-called prestress effect and are alloyed to the niobium and/or the copper or bronze, respectively. The alloy compositions disclosed in the European published patent application and the measures required for the production of superconductive wires, considerably reduce or even prevent the reduced maximum value of the critical current intensity which takes place primarily with high magnetic fields.
With the prior art methods for producing Nb.sub.3 Sn or V.sub.3 Ga multifilamentary superconductive wires, products are obtained which, at medium magnetic fields (about 12 Tesla), have critical current density (Jc) values around 10.sup.5 A/cm.sup.2 .